Released: September 12, 2002

West Nile Virus ‘Flood’ Likely in Weeks Ahead

MANHATTAN, Kan. – The U.S. Centers for Disease Control and Prevention (CDC) is expecting a flood of West Nile Virus reports in coming weeks, "as the transmission season peaks in different parts of the country."

Health officials nationwide are urging Americans to keep protecting themselves against mosquito bites, as well as eliminating pools of standing water. Still waters are the required maternity ward for the 26 mosquito species known to carry the disease between infected birds and other animals.

The incubation period for West Nile Virus in humans is 3-14 days after being bitten by an infected mosquito, said Ludek Zurek, entomologist with Kansas State University Research and Extension.

Since the disease first appeared on the continent three years ago in New York City, people age 50 and older have comprised the highest human risk group. However, West Nile has affected all age ranges, including children as young as 3, Zurek warned. It also has spread into all but six of the contiguous U.S. states and into several Canadian provinces.

The CDC’s West Nile Virus expert, Dr. Lyle Petersen, said research indicates one of every 150 West Nile Virus infections becomes serious – typically leading to meningitis or encephalitis. By Sept. 6, the CDC had received reports of 43 U.S. fatalities and 854 serious infections for the year. Thus, 120,000-plus Americans may already have had the disease, with many not noticing or thinking it was flu.

The virus probably is spreading so quickly because more than 110 bird species are known to get West Nile, Zurek said. (See related story.) Most birds survive, developing lifelong immunity. The virus remains active in their bloodstream for some time, however, and infected birds can serve as a source for spreading the disease. They just have to be bitten by mosquitoes or eaten by a predatory bird.

"The likely ways of transporting the virus internationally, as well as regionally, are migratory birds and mosquito-contaminated trade goods,"the entomologist said.

Most infected mammals show few symptoms or ill effects. Worldwide, the CDC’s reported list of confirmed West Nile Virus-positive animals includes horses, dogs, cats, domestic rabbits, squirrels, chipmunks, skunks, bats, white-tailed deer and black bears.

"Mammals appear to be ‘dead-end hosts.’ That means they can’t spread the infection to other animals, even if bitten by a mosquito," Zurek said.

Infected horses have the worst odds for surviving. Up to 40 percent die, compared to less than 1 percent of infected humans.

"Except for horses, you’ll probably never know if a domestic animal gets the disease. Its being infected certainly is no reason to destroy an animal, either, since it probably will recover," Zurek said.

The West Nile Virus vaccination released recently for horses may require six weeks to become totally effective. By that time, this year’s mosquito season could be over, he said. Even so, horse owners should contact their veterinarian for information and recommendations.

"The primary mosquito season in Kansas is May through mid-September, although in some years you’ll see several species as early as February and as late as November," the entomologist said. "The four species most likely to spread West Nile Virus in Kansas belong to the genus Culex – the members of which also overwinter as adults."

Zurek has sent in-depth information to every Kansas county’s K-State Research and Extension office, so the agents there can help local residents who want to know more about the disease, its spread, and prevention options available to individuals and farm operations. (That illustrated factsheet also is available on the Web at http://www.ksre.ksu.edu/westnilevirus/ and click on Kansas Insect Newsletter.)

"One of our most abundant mosquito species in Kansas breeds primarily in rural areas in the temporary to fairly permanent depressions found in pastures, ditches and springs," he pointed out.

Beyond that, mosquitoes can breed in livestock tanks and troughs as easily as in landscaping "water features." Sometimes they breed in livestock waste lagoons, as well as in lakes, ponds, pools, and either fresh or saltwater marshes.

Anywhere outdoor water gets trapped, mosquitoes can breed, Zurek said. This includes holes in trees and stumps, tarps on boats and swimming pools, depressions in rocks and concrete, and bowls for pets and bird baths. Soon it also will include leaf-clogged rain gutters.

"If she can get the blood meal needed to form eggs, each adult female mosquito will lay water rafts of 100 to 300 eggs every third night during the course of her several-week lifespan. For those eggs to become egg-laying adults usually takes just seven to 10 days," he said.

Public health officials and scientists have been asking residents to help them trace the disease’s spread, too. In general, they only want reports of dead crows and bluejays because those birds usually develop obvious symptoms. West Nile Virus attacks their kidneys and brains, often proving to be fatal, Zurek said.

Kansas’ toll-free West Nile Virus hotline,1-866-452-7810, is based in K-State’s Department of Entomology and still is taking reports from counties where the disease’s spread has yet to be confirmed. Kansans from known West Nile Virus counties should file dead-bird reports on the World Wide Web at http://www.nhm.ku.edu/birds.

So far, Kansas has recorded no serious human infections or deaths from the disease. But the virus has been implicated in the illness or death of horses and birds in more than 30 counties.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward, Communications Specialist
kward@oznet.ksu.edu
K-State Research& Extension News